1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention is directed to an electrochemical cell in which the current collector for at least one of the electrodes is of a metal foil containing openings or perforations that provide for ion migration from one side of the foil to the other. The openings may comprise 2% to 80% of the current collector surface area.
The perforated conductive foil is particularly useful for fabricating electrodes, whether of a positive or negative polarity for a primary or a secondary cell, from an electrode active material slurry. The active slurry is continuously coated onto one side of the current collector by applying a barrier to the opposite side of the foil. The barrier allows the slurry to fill the openings on the side being coated while simultaneously preventing migration of the active material to the opposite side thereof. Alternatively, no barrier is needed if the slurry viscosity is increased so that it does not pass through the openings before the electrode is dried.
Electrodes prepared in this manner are useful with only one side of the perforated current collector being slurry coated or, the second side of the current collector is slurry coated with the same or a different material than that supported on the first side. Such electrodes are particularly useful in a jellyroll type electrode assembly. It is also within the scope of the present invention to apply an electrode active material, being either the same or different than that on the first side of the current collector, to the second side by a pressing process.
2. Prior Art
An electrochemical cell typically contains current collectors that aid in the distribution of electrons to and from the counter electrode active materials. In conventional cells, a metal screen is used for this purpose. The problem is that it is difficult to continuously coat an electrode active material to a current collector screen. Therefore, each screen must be processed individually. This is labor-intensive, particularly if the cell being produced requires substantial electrode surface area.
For producing relatively large electrode volumes, it is more desirable to continuously coat active material onto the current collector. Such coating processes require a foil-type current collector. In some cases, however, it is also desirable to provide a means for ion migration from one side of the current collector to the other.
An example of an electrode where ion migration is important is described in U.S. Pat. No. 6,551,747 to Gan, which is assigned to the assignee of the present invention and incorporated herein by reference. This patent describes a lithium cell having a cathode of silver vanadium oxide (SVO) and fluorinated carbon (CFx) contacted to the opposite sides of the cathode current collector. During discharge, the SVO provides high current for pulse discharge applications while the CFx serves to re-charge the SVO once the load has been removed. Supporting the SVO and CFx on opposite sides of a perforated current collector facilitates this discharge interaction.
One aspect of this invention is, therefore, to coat an electrode active material onto a perforated foil containing a substantial area of openings or perforations through which ions may pass during cell discharge. The perforated foil serving as a current collector has from about 2% to about 80% open area and the electrode active coating is in the form of a slurry or paste contacted thereto. During the coating process, the open areas or perforations are blocked by applying a barrier to the opposite side of the foil. The barrier prevents the slurry from moving or leaking through the current collector openings and remains in place until the drying process is complete. An exemplary barrier is another foil devoid of openings. A second method of coating involves preparing a paste having a sufficiently high viscosity such that it cannot pass through the foil openings prior to drying.
Another aspect of the present invention involves pressure contacting a second, different electrode active material to the other side of the current collector after the first coating is complete. In this manner, two or more electrode active materials having different strengths and weaknesses share both ions and electrons during cell discharge. For example, a first active material having a relatively high capacity but a relatively low rate capability is contacted to one side of the perforated current collector foil, preferably as a coated slurry, and a second active material having a relatively low capacity but a relatively high rate capability is contacted to the other side. An example of this is a cathode having the configuration of: SVO/current collector/CFx. In such a construction, the high rate SVO material provides current for certain applications while the high capacity CFx material essentially re-charges the high rate material between loads.
A specific example of constructing an electrode having two active materials involves providing a cathode by first coating silver vanadium oxide onto the perforated current collector foil. The uncoated side of the current collector is then pressure contacted with CFx powder. If desired, a second assembly comprising a perforated current collector foil/SVO is positioned against the opposite side of the CFx to provide a three-layer xe2x80x9csandwichxe2x80x9d electrode with two intermediate current collectors. A schematic representation of this electrode construction has the following configuration: SVO/current collector/CFx/current collector/SVO. The outer SVO layers provide the current needed for high current applications while the CFx provides for higher capacity than SVO alone. Alternatively, the CFx material is the first active material coated and the SVO material is contacted to the current collector by some other means, such as by mechanical pressure.
In a still further embodiment of the present invention, both the first and second electrode active materials are slurry coated to the opposite sides of the perforated current collector. For example, in the case of a cathode comprising SVO and CFx, both active materials are provided in a slurry form and coated one after the other to the opposite sides of the perforated current collector. Which of them is coated first is not necessarily important.
When combined with a lithium anode, a cathode electrode having two different active materials, such as of the exemplary SVO and CFx, is particularly well suited for use in an implantable cardioverter defibrillator.
These and other aspects of the present invention will become increasingly more apparent to those of ordinary skill in the art by reference to the following description taken in conjunction with the accompanying drawings.